Conventionally, a method of impregnating a cathode with cesium oxide having a relatively small work function is proposed for improving the starting capability of a cold-cathode fluorescent lamp used, for example, as a back light in a liquid crystal display device (National Convention of Luminance Society (1992), Preliminary Report No. 42, Laid-open Japanese Patent Application No. (Tokkai Hei) 5-290811, U.S. Pat. No. 4,275,330). Furthermore, another method for improving starting property of an electrodeless fluorescent lamp is to fill the discharge space inside a bulb with a radioactive material (e.g., Gregory Zaslavsky, et al. Improved starting of the 100-W Metal Halide Lamp. JOURNAL of the Illuminating Engineering Society 19 (no.1): 76-83 (1990), Published Examined Japanese Patent Application No. (Tokko Sho) 60-34220). In this case, it is possible to obtain initial electrons quickly at the time of lighting by using electrons which arise in accordance with the decay of the radioactive material.
However, when a cathode is impregnated with cesium oxide as proposed above, a sintered electrode is impregnated with cesium oxide, so that the surface of the electrode must be polished after the impregnation process. Otherwise, since a large number of cesium ions, which are emitted through ion bombardment, falls off when a lamp is lit, the tube wall near the electrode is blackened during the initial stage. In addition, since the electrode surface is polished, the number of cesium oxide molecules is insufficient on the electrode surface, so that a starting auxiliary light source will be needed especially when it is lit in a dark environment. Examples of starting auxiliary light source include a UV enhancer disclosed in the publication mentioned above (Gregory Zaslavsky, et al.) and an igniter bulb disclosed in U.S. Pat. No. 4,359,668. However, when a starting auxiliary light source is used, the cost becomes higher and the system becomes considerably complicated. In addition, a tube wall near the electrode is blackened conspicuously even after being lit for a long period of time.
When a radioactive material is filled in discharge space inside a bulb as proposed above, it is necessary to restrict the amount of radiation as much as possible for safe handling of the radioactive material. As a result, it is difficult to obtain a fluorescent lamp with satisfactory starting ability particularly for general use.
Furthermore, another method is proposed in which a conventional product is combined with a a starting auxiliary light source, but this system is relatively complicated and increases costs.